This invention is directed in general to signal processing devices which simultaneously process two digital signals which may be asynchronous. Although the invention is not limited thereto, it is specifically applicable to use in digital echo cancellers and will be described in that context.
It is conventional in long distance telephone circuits to utilize a hybrid coupling to interconnect the long distance four-wire line and the local two-wire line. As is well known, however, the coupling provided by the hybrid is not ideal, and a portion of the received signal will pass through the hybrid to the transmit line and will be returned to the speaker as an "echo". An effective means for eliminating this echo is the "echo canceller" which is now quite well known in the art and is described, for example, in U.S. Pat. No. 4,064,379 to O. A. Horna entitled "Logarithmic Echo Canceller", or in the article entitled "Echo Canceller With Adaptive Transversal Filter", by O. A. Horna, Comsat Technical Review, Vol. 7, No. 2, Fall, 1977, pages 393-428. In principle, the echo canceller will monitor the incoming signal and predict the resulting echo, and this predicted echo is then subtracted from the signal present on the transmit line to hopefully cancel the echo signal.
FIG. 1 is a brief block diagram of a conventional echo cancellation system. The signal f.sub.si received on line 10 is provided to the two-wire line 12 via the hybrid coupler 14 which includes an echo path by which a portion of the received signal f.sub.si is passed to the transmit side of the hybrid and would thus be returned to the distant party over the transmit line 16. The echo canceller essentially comprises an adaptive finite impulse response filter (AFIRF) 18 which predicts the echo at any given time based upon recent samples of the received signal, and the predicted echo is subtracted in adder 20 from the output signal f.sub.so of the hybrid. In the above-mentioned patent to Horna, the AFIRF receives its incoming signal input from amplifier 201 and its error signal input from sample-and-hold 222 and provides its output from D/A circuit 219. If the echo is correctly predicted, the subtraction of the echo prediction will eliminate entirely any echo component in the output signal f.sub.so. Assuming that the near talker 22 is silent, and further assuming an ideal noise-free communications channel, there should be no signal on line 16 at the output of adder 20, and any signal which is present at the output of adder 20 will be fed back to the AFIRF to adjust the prediction algorithm. In reality, the echo canceller may include circuitry for compensating for both noise and for handling "double-talk" situations, as is well known in the art, but a description of these features is not necessary for a complete understanding of the present invention.
The difference between the true and computed echo, i.e., the "error" is used to compute an improved model of the unit impulse response stored in the memory of the AFIRF.
In many applications, the signals on the input and output lines 10 and 16 are analog, and the signals are sampled and the samples converted to digital form within the AFIRF. The prediction signal processing is then carried out digitally and the predicted echo is then converted to analog form for subtraction from the true echo. In these cases, it is a simple matter to sample the receive and send lines synchronously with the internal clock of the digital portion of the AFIRF.
A significant problem arises, however, if the echo canceller is utilized in a circuit which carries digitally encoded signals. In such a case, circuits 203, 204 and 219 of the Horna patent would be dispensed with. The send and receive signals are in the form of digital bit streams, and ideally both would have the same bit rate. However, due to the influence of a long distance transmission media, e.g., doppler shift of a satellite circuit or frequency tolerances of local clocks of A/D encoders, the bit rate of the receive signal f.sub.si can differ from the bit rate of the send signal f.sub.so by as much as .+-.50 parts per million. With an 8 kHz sampling rate, the sampling rates seen on the send and receive lines can differ by 0.4 Hz which will manifest itself as a 0.8.pi./second "phase roll". This phase roll can substantially reduce the effectiveness of the echo canceller, as is well known in the art and as is described in the above-cited paper by Horna.
Since digital systems may transmit not only digitally encoded speech and video signals but also computer data, the rate of the digital bit stream at either the receive or send side cannot be altered without risking data loss. This substantially limits the possible solutions to the problem of asynchronous data rates. One possible solution is described in U.S. Pat. No. 4,074,086 to Falconer et al entitled "Joint Adaptive Echo Canceller and Equalizer for Two-wire Full-duplex Data Transmission". The technique disclosed therein is based on a special property of the equalizer, i.e., that the location of the most significant coefficients h in the coefficient memory can be fairly well predicted. When the receive and send bit stream are out of synchronism by nearly one-half of the sampling period, one least significant coefficient h is truncated or read twice from the memory to establish synchronism. This method is difficult to implement in a telephone echo canceller, since the location of the most significant coefficients changes with every connection.